So far nothing alarming has turned up, according to Dvorak. "The hematological parameters are normal," he said.

FIFA is building up a biological passport on all players.

It has taken at least two test samples from most players while some have given up to four.

So far teams have made the doping investigators welcome. "They understand what we are doing and they consider that kind of examination as part of their professional life," said Dvorak.

The FIFA tests look for discrepancies in haemoglobins and red cells that could indicate EPO doping or other banned efforts to boost endurance. These kinds of tests are already used in cycling and athletics.

Football's governing body also checks hormone levels and for anabolic steroids in the urine.